Process for producing hydrocarbons with low sulphur and nitrogen contents

ABSTRACT

A process for desulphurizing a gasoline feed comprising at least 150 ppm by weight of sulphur-containing compounds using a hydrodesulphurization catalyst is characterized in that said feed undergoes prior denitrogenation treatment under conditions such that the amount of nitrogen-containing compounds present in said feed when it is brought into contact with said hydrodesulphurization catalyst does not exceed 150 ppm by weight.

[0001] The present invention relates to a process for producing hydrocarbons with a low sulphur content. Said hydrocarbon fraction contains an olefin fraction that generally exceeds 5% by weight, usually 10% by weight, a sulphur content of more than 100 ppm by weight and a nitrogen content of more than 20 ppm by weight. The process allows all of a gasoline cut containing sulphur to be upgraded, reducing the sulphur content of said gasoline cut to very low levels, without reducing the gasoline yield, and minimizing the reduction in the octane number during said process. The invention is of particular application when the gasoline to be treated is a cracked gasoline containing more than 300 ppm by weight or more than 500 ppm by weight of sulphur, and a nitrogen content that is generally more than 50 ppm by weight, or more than 100 ppm by weight, preferably more than 150 ppm by weight, or 200 ppm by weight or more.

[0002] Future regulations regarding vehicle fuels will require a substantial reduction in the sulphur content of those fuels, in particular gasoline. That reduction is particularly aimed at limiting the sulphur dioxide content and the nitrogen content in particular in vehicle exhausts. Current specifications regarding sulphur contents are of the order of 150 ppm by weight and will be reduced in the future to below 10 ppm via a limit of 30 ppm by weight. The change in the sulphur content specifications necessitates the development of novel processes for severe desulphurization of gasoline.

[0003] The principal sources of sulphur in gasoline bases are cracked gasolines, principally the gasoline fraction deriving from a process for catalytic cracking of a residue from atmospheric distillation or vacuum distillation of crude oil. The gasoline fraction derived from catalytic cracking, which represents an average of 40% of gasoline bases, contributes more than 90% of the sulphur in that gasoline. As a result, the production of low sulphur gasoline requires a step for desulphurizing catalytically cracked gasoline. That desulphurization is conventionally achieved by one or more steps for bringing the sulphur-containing compounds contained in said gas into contact with a gas that is rich in hydrogen in a hydrodesulphurization process.

[0004] Further, the octane number of said gasolines is very closely linked to their olefin content. Preserving the octane number of said gasolines thus necessitates limiting reactions transforming olefins to paraffins, which reactions are inherent to hydrodesulphurization processes.

[0005] When the gasoline is desulphurized conventionally, the olefin saturation reactions occurring in parallel with the reactions transforming sulphur-containing compounds into H₂S lead to a substantial octane number drop. Within the context of restriction to sulphur specifications in gasoline, such processes result in severe drops in octane number. A variety of solutions have been proposed to selectively eliminate the sulphur-containing compounds from the gasoline by limiting undesirable olefin hydrogenation reactions, generally determined by the skilled person as the degree of olefin saturation at the reactor outlet. However, the impact of nitrogen-containing compounds on the activity and/or selectivity of gasoline hydrodesulphurization catalysts is practically unknown.

[0006] However, in the case of hydrotreating middle distillates (gas oil and kerosene), the presence of nitrogen-containing compounds is known to inhibit hydrodesulphurization reactions. As an example, a process for producing middle distillates with a low sulphur content has been proposed, comprising a step for eliminating basic nitrogen before the hydrotreatment step (Proc Int Conf Stab Handl Liq Fuels, 7^(th) Meeting, 2000, Vol 1, 153-163). This step for eliminating basic nitrogen is stated to be necessary to obtaining high degress of desulphurization. However, the technical problems posed by gasoline hydrodesulphurization appear to differ from those posed by hydrodesulphurization of middle distillates, in particular due to difficulties inherent to the presence of unsaturated compounds in gasolines and to the specific problem of the loss of octane number described above.

[0007] In the particular case of gasoline, U.S. Pat. No. 6,120,679 describes, in contrast, a method for preparing hydrodesulphurization catalysts based on a step for pre-treating the catalysts with a nitrogen-containing compound (pyridine).

[0008] Studies carried out by the Applicant concerning the present application have demonstrated that the presence of nitrogen in a substantial quantity, i.e., in amounts higher than those previously described, have a non negligible effect on the activity of the gasoline hydrodesulphurization catalyst and in particular, the presence of basic nitrogen-containing compounds such as pyridine beyond a certain threshold degrades not only the activity but also the selectivity of the hydrodesulphurization catalysts. In particular, the Applicant has discovered that eliminating certain proportions of said nitrogen-containing compounds has a very substantial effect on the decomposition of sulphur-containing compounds by the catalyst, but a relatively limited effect on the hydrogenation of unsaturated compounds. In one possible application of the present invention, for the same degree of saturation of olefins at the reactor outlet and for the same operating temperature for the hydrodesulphurization reactor, it is possible to increase the catalyst activity by prior reduction in the nitrogen-containing compounds present in the gas. In a further application of the present invention, eliminating basic nitrogen-containing compounds prior to hydrodesulphurization can limit the degree of olefin saturation for a fixed sulphur content at the reactor outlet. The present invention is thus of particular application to the treatment of gasoline cuts with a high nitrogen-containing compound content.

[0009] In general, the present invention relates to a process that can achieve at least one of the following advantages and preferably all simultaneously:

[0010] satisfy future specifications on vehicle gasoline, i.e., sulphur contents of the order of 50 ppm or less than 10 ppm depending on the State;

[0011] limit the nitrogen content in the gasoline;

[0012] control olefin hydrogenation processes during said process;

[0013] thereby limit the octane number loss linked to hydrodesulphurization processes;

[0014] maximize the service life of hydrodesulphurization catalysts by employing hydrodesulphurization reactors at lower temperatures.

[0015] In summary, the present desulphurization process proposes a solution to obtaining high degrees of desulphurization while limiting the octane number loss due to olefin hydrogenation. This results in the production of a low sulphur gasoline with a high octane number.

[0016] The present invention provides a process for desulphurizing a gasoline feed comprising at least 150 ppm by weight of sulphur-containing compounds using a hydrodesulphurization catalyst, characterized in that said feed undergoes prior denitrogenation treatment under conditions such that the amount of nitrogen-containing compounds present in said feed when it is brought into contact with said hydrodesulphurization catalyst does not exceed 150 ppm by weight.

[0017] In one possible implementation of the invention, the denitrogenation treatment is carried out immediately prior to said contact (hydrodesulphurization).

[0018] In a first implementation, for example when said treatment is carried out immediately prior to said contact (step e) and possibly f)), at least one step selected from the group constituted by:

[0019] a) selective hydrogenation of the dienes contained in the feed;

[0020] b) transformation of light sulphur-containing compounds contained in the feed;

[0021] c) separation of said feed into at least two fractions including:

[0022] a light fraction containing a minor portion of the sulphur-containing compounds;

[0023] a heavy fraction containing a major portion of the sulphur-containing compounds;

[0024] is carried out prior to said denitrogenation temperature (step d)).

[0025] Advantageously, said contact (step e) and possibly step f)) is preferably carried out with at least the heavy fraction from step c).

[0026] In an alternative implementation, at least one step selected from the group constituted by:

[0027] a) selective hydrogenation of the dienes contained in the feed;

[0028] b) transformation of light sulphur-containing compounds contained in the feed;

[0029] c) separation of said feed into at least two fractions including:

[0030] a light fraction containing a minor portion of sulphur-containing compounds;

[0031] a heavy fraction containing a major portion of the sulphur-containing compounds;

[0032] is carried out between said denitrogenation treatment (step d)) and said contact (step e) and possibly step f)).

[0033] Advantageously, said contact (hydrodesulphurization) is preferably carried out with at least said heavy fraction from step c).

[0034] Preferably, said contact is made in at least two steps e) and f), regardless of the envisaged implementation.

[0035] In general, said hydrodesulphurization catalyst comprises at least one element from group VIII of the periodic table, and advantageously, said hydrodesulphurization catalyst comprises at least one element from group VIB of the periodic table.

[0036] Preferably, said group VIII element is selected from the group constituted by nickel and cobalt and said at least one group VIB element is selected from the group constituted by molybdenum and tungsten.

[0037] The conditions for said contact are generally as follows: a temperature in the range 200° C. to 450° C., a pressure in the range 1 to 3 MPa, an hourly space velocity in the range 1 h⁻¹ to 10 h⁻¹, and a H₂/HC ratio (ratio of hydrogen to hydrocarbons, expressed in litres per litre) in the range 50 l/l to 500 l/l.

[0038] The present process can advantageously be applied to gasoline from catalytic cracking or from cokefaction of a heavy hydrocarbon feed or from steam cracking.

[0039] The invention will be better understood from the following description of an implementation given purely by way of illustration and which is not in any way limiting.

[0040] In a preferred but not obligatory implementation of the invention, the feed to be desulphurized is optionally pre-treated in a concatenation of reactors for selective diolefin hydrogenation (step a) and for rendering light-sulphur-containing compounds heavier (step b)). The feed that has optionally been pre-treated is then distilled and fractionated into at least two cuts (step c)): a light gasoline that is depleted in sulphur and rich in olefins, and a heavy gasoline that is rich in sulphur and depleted in olefins. The light fraction from the three preceding steps generally contains less than 100 ppm of sulphur, preferably less than 50 ppm of sulphur, and highly preferably, less than 20 ppm of sulphur, and in general does not need subsequent treatment prior to its incorporation as a gasoline base. The heavy fraction from the three preceding steps, which contains the major portion of the sulphur, is treated using the process of the present invention. This preferred implementation has the advantage of further minimizing the octane number loss as light olefins containing 5 carbon atoms, which are readily hydrogenated, are not sent to the hydrodesulphurization section.

[0041] Step a) is optional and is principally intended to eliminate the diolefins present in the gasoline. This step can maximize the service life of catalysts used in the downstream steps. Steps b) and c) are also optional, but if they are carried out prior to step e), they can minimize the overall octane number loss in the process.

[0042] Denitrogenation step d) is carried out before contact with the hydrodesulphurization catalyst (steps e) and/or f)) or before at least one of steps a), b) and/or c), so that the amount of nitrogen-containing compounds does not exceed 150 ppm (expressed by weight), preferably 125 ppm, more preferably 100 ppm.

[0043] The process of the invention comprises at least the two steps d) and e). Step d) corresponds to a step for at least partial elimination of the nitrogen contained in the gasoline; step e) corresponds to a step for hydrotreatment of the pre-treated gasoline.

[0044] In general, the experimental conditions of these pre-treatment, denitrogenation or hydrodesulphurisation steps a) to f) are as follows:

[0045] 1) Selective Hydrogenation (Step a)):

[0046] This optional pre-treatment step for the gasoline to be desulphurized is intended to at least partially eliminate the diolefins present in the gasoline. Diene hydrogenation is an optional but advantageous step, which can eliminate the vast majority of the dienes present in the cut to be treated prior to hydrotreatment. Diolefins are precursors to gums, which polymerise in the hydrotreatment reactors and limit their service life.

[0047] This step generally takes place in the presence of a catalyst comprising at least one group VIII metal, preferably selected from the group constituted by platinum, palladium and nickel, and a support. As an example, it is possible to use a catalyst containing 1% to 20% by weight of nickel deposited on an inert support such as alumina, silica, silica-alumina, a nickel aluminate or a support containing at least 50% alumina. This catalyst operates at a pressure of 0.4 to 5 MPa, at a temperature of 50° C. to 250° C., with an hourly space velocity of the liquid of 1 h⁻¹ to 10 h⁻¹. A further group VIB metal such as molybdenum or tungsten can be combined therewith to form a bimetallic catalyst. This group VIB metal, if combined with a group VIII metal, will be deposited in an amount of 1% by weight to 20% by weight.

[0048] As to the operating conditions, usually, we operate under pressure in the presence of a quantity of hydrogen that is in slight excess with respect to the stoichiometric value necessary to hydrogenate the diolefins. The hydrogen and the feed to be treated are injected as upflows or downflows into a reactor, preferably with a fixed catalyst bed. The temperature is most generally in the range 50° C. to 300° C., preferably in the range 80° C. to 250° C., more preferably in the range 120° C. to 210° C.

[0049] Most generally, the pressure is 0.4 to 5 MPa, preferably more than 1 MPa. An advantageous pressure is in the range 1 to 4 MPa, limits included.

[0050] Under these conditions, the space velocity is of the order of 1 h⁻¹ to 12 h⁻¹, preferably of the order of 4 h⁻¹ to 10 h⁻¹.

[0051] The light fraction of the catalytically cracked gasoline cut can contain up to a few % by weight of diolefins. After hydrogenation, the diolefins content is reduced to less than 3000 ppm, or less than 2500 ppm and more preferably less than 1500 ppm. In some cases, a content of less than 500 ppm can be achieved. The diene content after selective hydrogenation can even be reduced to less than 250 ppm.

[0052] Concomitantly with the selective diolefin hydrogenation reaction, the external double bond of the olefins leads to the formation of internal olefins. This isomerization results in the formation of olefins that are more resistant to saturation with hydrogen and to s slight increase in octane number (or a compensation in octane number due to the slight olefin loss). This is due to the fact that internal olefins have an octane number that is generally higher than that of terminal olefins.

[0053] In one implementation of the invention, the diene hydrogenation step is carried out in a catalytic hydrogenation reactor that comprises a catalytic reaction zone preferably traversed by the entire feed and by the quantity of hydrogen necessary to carry out the desired reactions.

[0054] Certain nitrogen-containing compounds are also transformed during this step. This is the case, for example, with slightly basic nitriles which, through hydrogenation, are transformed into amines which are more basic.

[0055] 2) Transformation of Light Sulphur-Containing Compounds (Step b)):

[0056] This optional step consists of transforming light saturated sulphur-containing compounds, i.e., compounds with a boiling point that is lower than that of thiophene, into saturated sulphur-containing compounds with a boiling point that is higher than that of thiophene. Said light sulphur-containing compounds are typically mercaptans containing 1 to 5 carbon atoms, CS₂ and sulphides containing 2 to 4 carbon atoms. This transformation is preferably carried out over a catalyst comprising at least one group VIII element (groups 8, 9 and 10 of the new periodic table) on an alumina, silica or silica-alumina or nickel aluminate type support. The choice of catalyst is made so as to promote the reaction between light mercaptans and olefins, which results in mercaptans or sulphides with boiling points that are higher than thiophene.

[0057] This optional step can possibly be carried at the same time as step a), in the same reaction bed and with the same catalyst. As an example, it may be particularly advantageous to operate, during the diolefin hydrogenation, under conditions such that at least a portion of the mercaptans are transformed.

[0058] In this case, the temperatures are generally in the range 100° C. to 300° C., preferably in the range 150° C. to 250° C. The H₂/feed ratio is adjusted to between 1 and 20 litres per litre, preferably to between 3 and 15 litres per litre. The space velocity is generally in the range 1 h⁻¹ to 10 h⁻¹, preferably in the range 2 h⁻¹ to 6 h⁻¹, and the pressure is in the range 0.5 to 5 MPa, preferably in the range 1 to 3 MPa.

[0059] The nitrogen-containing compounds present in the gas are also partially rendered heavier during this step. The inventors have discovered that the nitrogen-containing compounds present in the IP (initial point) fraction −60° C. were transformed into heavier nitrogen-containing compounds with a boiling point of more than 60° C. Thus, step b) renders possible separation of a portion of the nitrogen-containing compounds from the IP-60° C. fraction.

[0060] 3) Separation of Gasoline into at Least Two Fractions (Step c)):

[0061] This optional step, carried out after steps a) and b), can produce a light desulphurized gasoline, generally containing less than 5 ppm of mercaptans. During this sep, the gasoline is fractionated into at least two fractions:

[0062] a light fraction containing a limited residual sulphur content, preferably less than about 50 ppm, more preferably less than about 20 ppm, highly preferably less than about 10 ppm, and which enables said cut to be used without carrying out (an)other treatment(s) aimed at reducing the sulphur content; this light fraction is generally depleted in light nitrogen-containing compounds;

[0063] a heavy fraction in which the major portion of the sulphur initially present in the feed i.e., all of the sulphur which is not found in the light gasoline, is concentrated.

[0064] Said separation is preferably carried out using a conventional distillation column. This fractionation column can separate a light fraction of the gasoline containing a small fraction of sulphur from a heavy fraction preferably containing the major portion of the sulphur initially present in the initial gasoline.

[0065] The light gasoline obtained following separation generally contains at least all of the olefins containing five carbon atoms, preferably compounds containing five carbon atoms and at least 20% of olefins containing six carbon atoms. Generally, this light fraction obtained after steps a) and b) has a low sulphur content, i.e., it is not in general necessary to treat the light cut before using it as a fuel.

[0066] 4) Elimination of Nitrogen from Gasoline: Step d):

[0067] The nitrogen-containing compounds present in gasoline are principally from the following families: nitrites, amines, pyrroles, pyridines and anilines. These compounds are generally present in the gasoline in an amount of 20 to 400 ppm. Most of these compounds are basic; thus, they can be eliminated by separation in an acidic medium. The step for eliminating nitrogen from the gasoline can thus consist of washing the gasoline with an aqueous solution containing an acid compound. Examples of acids that can be cited are phosphoric acid, sulphuric acid, hydrochloric acid and formic acid. Any type of acid that is soluble in water and has sufficient acidity to protonate nitrogen can be used for this operation. This operation is carried out by bringing the gasoline to be treated into contact with the acid, for example in a washing column. The washing conditions are optimized so that the gasoline that is recovered contains less than 150 ppm of nitrogen, preferably less than 100 ppm by weight, and more preferably less than 50 ppm of nitrogen, or less than 20 ppm.

[0068] Step d) can also be accomplished by treating the gasoline on a solid with a sufficient Lewis or Brönsted acidity to fix the nitrogen-containing compounds. Examples of solids that can be used are ion exchange resins, strong acids on mineral supports such as phosphoric acid on silica, or silica aluminas in the zeolitic or amorphous form. This list is only given by way of illustration, and the scope of the present invention encompasses the use of any other known technique for eliminating all or a portion of the nitrogen-containing compounds present in a hydrocarbon fraction. The gasoline traverses a guard mass that is in general use in a fixed bed, the basic nitrogen-containing compounds are protonated and become fixed on the mass. Once saturated, the mass can be regenerated, or more simply, replaced with a fresh mass.

[0069] The choice of mass, its length of use and the operating conditions are optimized so that the gasoline produced during step d) contains less than 150 ppm of nitrogen, or 100 ppm of nitrogen, preferably less than 50 ppm of nitrogen, more preferably less than 20 ppm of nitrogen. In a further implementation, the choice of mass, its service life and the operating conditions are optimized so that at least 50%, preferably 70% and more preferably at least 90% of the nitrogen-containing compounds are eliminated during this step.

[0070] In an advantageous implementation of the invention, step a) is carried out before step d). Certain nitrogen-containing compounds such as nitrites are transformed during step a) to form the corresponding amines. The observed reaction is as follows:

CH₃—CH₂—CN+2H₂→CH₃—CH₂—CH₂—NH₂

[0071] As amines are more basic than nitrites, their extraction during step d) will be facilitated.

[0072] Step d) can also include separation, generally by distillation, of the gasoline to be treated. The basic compounds present in the cracked gasoline are concentrated in the heavy fraction of the gasoline. Said heavy fraction is eliminated by distillation, and can thus at least partially eliminate the basic nitrogen-containing compounds. In this case, step d), which consists of distillation, produces at least two fractions:

[0073] the light fraction, which concentrates the olefins and which is depleted in nitrogen;

[0074] the heavy fraction, which concentrates the basic nitrogen and the aromatics and which is depleted in olefins.

[0075] 5) Gasoline Hydrodesulphurization: Step e)

[0076] The hydrodesulphurization step (step e)) consists of passing the gas to be treated, in the presence of hydrogen, over a hydrodesulphurization catalyst at a temperature in the range 200° C. to 350° C., preferably in the range 250° C. to 320° C. and at a pressure in the range 1 to 3 MPa, preferably in the range 1.5 to 2.5 MPa. The liquid space velocity is generally in the range 1 h⁻¹ to 10 h⁻¹, preferably in the range 2 h⁻¹ to 5 h⁻¹; the H₂/HC ratio is 50 litres/litre (l/l) to 500 l/l, preferably in the range 100 l/l to 450 l/l, and more preferably in the range 150 l/l to 400 l/l. The H₂/HC ratio is the ratio between the hydrogen flow rate at 1 atmosphere and at 0° C. and the hydrocarbon flow rate. Under these conditions, the reaction takes place in the gas phase. The operating conditions during this step are adjusted as a function of the characteristics of the feed to be treated, to accomplish the desired degree of desulphurization. The effluents from said hydrodesulphurization step are partially desulphurized gasoline, residual hydrogen and the H₂ S produced by decomposition of the sulphur-containing compounds.

[0077] The catalysts used during step e) comprise at least one group VIII element and/or at least one group VIB element on a suitable support.

[0078] The amount of group VIII metal, expressed as the oxide, is generally in the range from 0.5% to 15% by weight, preferably in the range 1% to 10% by weight. The amount of group VIB metal is generally in the range 1.5% to 60% by weight, preferably in the range 3% to 50% by weight.

[0079] The group VIII element, when present, is preferably cobalt, and the group VIB element, when present, is generally molybdenum or tungsten. The catalyst support is normally a porous solid such as an alumina, a silica-alumina, or other porous solids, such as magnesia, silica or titanium oxide, used alone or as a mixture with alumina or silica-alumina. To minimize hydrogenation of the olefins present in the heavy gasoline, it is preferable to use a catalyst in which the density of the molybdenum, expressed as the % by weight of MoO₃ per unit surface area, is more than 0.07 and preferably more than 0.10. The catalyst of the invention preferably has a specific surface area of less than 200 m²/g, more preferably less than 180 m²/g, and highly preferably less than 150 m²/g.

[0080] The catalyst used is preferably in an at least partially sulphurized form. The sulphur or sulphur-containing compound can be introduced ex situ, i.e., outside the reactor in which the process of the invention is carried out, or in situ, i.e., in the reactor used for the process of the invention. Sulphurization consists of passing a feed containing at least one sulphur-containing compound, which once decomposed fixes sulphur on the catalyst. This feed can be gaseous or liquid, for example hydrogen containing H₂S, or a liquid containing at least one sulphur-containing compound.

[0081] 6) Gasoline Hydrodesulphurization: Step f)

[0082] Hydrodesulphurization step e) can be followed by a supplemental step aimed at improving the final degree of desulphurization. This step is compulsory after step e) and can be carried out with or without intermediate H₂S elimination. Step f) comprises at least one step for decomposing saturated sulphur-containing compounds deriving from step e). Said sulphur-containing compounds are transformed into H₂S over a catalyst and under conditions such that the olefins are only very slightly hydrogenated. The degree of hydrogenation (saturation) of olefins in this step is generally less than 20%, and preferably less than 10%.

[0083] This hydrodesulphurization step (step f)) generally consists of passing the gasoline to be treated, in the presence of hydrogen, over a hydrodesulphurization catalyst, at a temperature in the range 250° C. to 450° C., preferably in the range 300° C. to 360° C. and at a pressure in the range 1 to 3 MPa, preferably in the range 1.5 to 2.5 MPa. The liquid space velocity is generally in the range 1 h⁻¹ to 10 h⁻¹, preferably in the range 1 h⁻¹ to 5 h⁻¹; the H₂/HC ratio is in the range 50 litres/litre (l/l) to 500 l/l, preferably in the range 100 l/l to 450 l/l, and more preferably in the range 150 l/l to 400 l/l. Under these conditions, the reaction takes place in the gas phase. The operating conditions during this step are thus adjusted as a function of the characteristics of the feed to be treated in order to reach the desired degree of desulphurization.

[0084] The catalyst used during step e) comprises at least one group VII element selected from the group constituted by nickel, cobalt, iron, molybdenum and tungsten.

[0085] The amount of group VIII metal, expressed as the oxide, is generally in the range 1% to 60% by weight, preferably in the range 1% to 40% by weight.

[0086] The catalyst support is normally a porous solid such as an alumina, a silica-alumina or other porous solids such as magnesia, silica or titanium oxide, used alone or as a mixture with alumina or silica-alumina. The catalyst of the invention preferably has a specific surface area in the range 25 to 350 m²/g.

[0087] The catalyst is preferably at least partially in the sulphurized form. The sulphur or a sulphur-containing compound can be introduced ex situ, i.e., outside the reactor in which the process of the invention is carried out, or in situ, i.e., in the reactor used for the process of the invention. Sulphurization consists of passing a feed containing at least one sulphur-containing compound, which once decomposed fixes sulphur on the catalyst. This feed can be gaseous or liquid, for example hydrogen containing H₂S, or a liquid containing at least one sulphur-containing compound.

[0088] The importance and advantages of the present invention will be demonstrated by comparing Example 1 in accordance with the prior art with Example 2, in accordance with the invention.

EXAMPLE 1

[0089] (Prior Art)

[0090] Example 1 concerns a desulphurization process with no preliminary nitrogen elimination.

[0091] A hydrodesulphurization catalyst A was obtained by impregnating a transition alumina in the form of beads with a specific surface area of 130 m²/g and with a pore volume of 1.04 ml/g, with an aqueous solution containing molybdenum and cobalt in the form of ammonium heptamolybdate and cobalt nitrate. The catalyst was then dried and calcined in air at 500° C. the amount of cobalt and molybdenum in this sample was 3% of CoO and 10% of MoO₃.

[0092] 100 ml of catalyst A was placed in a hydrodesulphurization tube reactor using a fixed bed. The catalyst was initially sulphurized by treatment for 4 hours at a pressure of 3.4 MPa at 350° C., in contact with a feed constituted by 2% of sulphur in the form of dimethyldisulphide in n-heptane.

[0093] The treated feed was a catalytically cracked gasoline with an initial boiling point of 50° C. and an end point of 225° C. The sulphur content was 1450 ppm by weight and its bromine index (BrI) was 69 g/100 g. This gasoline had a nitrogen content of 180 ppm of nitrogen including 165 ppm of basic nitrogen (the term “basic nitrogen” means the nitrogen included in compounds comprising a nitrogen-containing group with a basic nature). The total nitrogen was assayed using American standard method ASTM 4629, and the basic nitrogen was assayed using ASTM 4739.

[0094] This feed was treated over catalyst A, at a pressure of 2 MPa, a H₂/HC ratio of 300 l/l and a HSV of 2 h⁻¹. Table 1 shows the influence of temperature on the degress of olefin desulphurization and saturation. TABLE 1 Sulphur content BrI of in desulphurized Degree of desulphurized Degree of olefin Temperature gasoline desulphurization gasoline saturation (° C.) (ppm by weight) (HDS-%) (g/100 g) (HDO-%) 280 292 79.9 49.2 28.7 290 165 88.6 45.6 33.9 300 108 92.6 38.97 43.6

[0095] In this example, it appears to be difficult to achieve low sulphur contents in the effluents for this feed. At 300° C., the sulphur content in the effluent was more than 100 ppm for a degree of olefin saturation of close to 44%.

EXAMPLE 2

[0096] In Accordance with the Invention

[0097] Example 2 was carried out in accordance with the invention, i.e., most of the basic nitrogen-containing compounds were eliminated during an acid washing step carried out prior to desulphurization.

[0098] The feed that was treated was the same as that of Example 1. This gasoline contained 180 ppm of nitrogen, including 165 ppm of basic nitrogen. 50 kg of this gasoline was mixed, in a batch reactor, with 100 kg of a 10% by weight concentrated sulphuric acid solution in distilled water. The mixture was stirred for 15 minutes then allowed to settle. The aqueous phase in the lower portion of the reactor was drawn off. The remaining gasoline was washed with 50 kg of distilled water. After settling, the water was separated from the gasoline.

[0099] An analysis showed that the gasoline produced had a nitrogen content of 12 ppm, including 0 ppm of basic nitrogen.

[0100] The reactor used in Example 1 was charged with fresh catalyst A and sulphurized using the same procedure as that described in Example 1.

[0101] This feed was treated over catalyst A, at a pressure of 2 MPa, a H₂/HC ratio of 300 l/l and a HSV of 2 h⁻¹. The operating conditions applied in Example 2 were identical to the operating conditions of Example 1. Table 2 shows the influence of temperature on the degrees of olefin desulphurization and saturation. TABLE 2 Sulphur content BrI of in desulphurized Degree of desulphurized Degree of olefin Temperature gasoline desulphurization gasoline saturation o (° C.) (ppm by weight) (HDS-%) (g/100 g) (HDO-%) 280 160 89.0 48.2 30.1 290 97 93.3 43.1 37.5 300 59 95.9 37.4 45.8

[0102] Under the same operating conditions, the degree of desulphurization achieved in the process of the invention was higher than that of Example 1. In contrast, the degrees of olefin saturation were comparable. This means that a desulphurization process carried out in accordance with the invention can increase the selectivity of the catalyst used: the loss of olefins and thus the octane number (measured at constant degree of desulphurization) are lower when the gasoline is at least partially freed from nitrogen-containing compounds before desulphurization than when treated directly.

[0103] The reduction in the amount of nitrogen in the gasoline prior to hydrodesulphurization also causes a substantial improvement in catalyst activity. This increase can, for example, minimize deactivation phenomena and maximize the service life of hydrodesulphurization catalysts by using lower operating temperatures.

[0104] The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

[0105] The entire disclosures of all applications, patents and publications, cited herein and of corresponding French application No. 02/07.054, filed Jun. 7, 2002 are incorporated by reference herein.

[0106] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. A process for desulphurizing a gasoline feed comprising at least 150 ppm by weight of sulphur-containing compounds using a hydrodesulphurization catalyst, characterized in that said feed undergoes prior denitrogenation treatment under conditions such that the amount of nitrogen-containing compounds present in said feed when it is brought into contact with said hydrodesulphurization catalyst does not exceed 150 ppm by weight.
 2. A process according to claim 1, in which said denitrogenation treatment is carried out immediately prior to said contact.
 3. A process according to claim 1, in which at least one step selected from the group constituted by: a) selective hydrogenation of the dienes contained in the feed; b) transformation of light sulphur-containing compounds contained in the feed; c) separation of said feed into at least two fractions including: a light fraction containing a minor portion of the sulphur-containing compounds; a heavy fraction containing a major portion of the sulphur-containing compounds; is carried out between said denitrogenation treatment (step d)) and said contact with the hydrodesulphurization catalyst (step e) and possibly step f))
 4. A process according to claim 3, in which said contact is made with at least the heavy fraction from step c).
 5. A process according to claim 2, in which at least one step selected from the group constituted by: a) selective hydrogenation of the dienes contained in the feed; b) transformation of light sulphur-containing compounds contained in the feed; c) separation of said feed into at least two fractions including: a light fraction containing a minor portion of the sulphur-containing compounds; a heavy fraction containing a major portion of the sulphur-containing compounds; is carried out prior to said denitrogenation temperature (step d)).
 6. A process according to claim 5, in which contact with the hydrodesulphurization catalyst is made with at least the heavy fraction from step c).
 7. A process according to one of the preceding claims, in which said contact is made in at least two steps e) and f).
 8. A process according to one of the preceding claims, in which said hydrodesulphurization catalyst comprises at least one element from group VII of the periodic table.
 9. A process according to one of the preceding claims, in which said hydrodesulphurization catalyst comprises at least one element from group VIB of the periodic table.
 10. A process according to claim 8 or claim 9, in which said catalyst comprises at least one element from group VIII of the periodic table selected from the group constituted by nickel and cobalt, and at least one group VIB element selected from the group constituted by molybdenum and tungsten.
 11. A process according to one of the preceding claims, in which said contact is made at a temperature in the range 250° C. to 350° C., a pressure in the range 1 to 3 MPa, an hourly space velocity in the range 1 h⁻¹ to 10 h⁻¹, and a H₂/HC ratio in the range 50 l/l to 500 l/l.
 12. Application of the process according to one of the preceding claims, to gasoline from catalytic cracking or from cokefaction of a heavy hydrocarbon feed or from steam cracking. 